There exist a number of different kinds of methods for measuring the temperature of a product.
U.S. Pat. No. 4,499,357 discloses an electronically controlled cooking apparatus in which an article to be heated is heated for cooking through measurement of temperature thereof by an infrared sensor. The infrared sensor is arranged to detect surface temperature of the article to be heated in electronically controlled cooking apparatuses of this kind. A problem with heating of food products is that the surface often reaches a high temperature fairly quickly whereas it often takes considerable time before the inside of the product reaches the required temperature. U.S. Pat. No. 4,499,357 addresses this problem by introducing a predetermined minimum heating time period being set such that heating of the article to be heated is unconditionally continued during the predetermined minimum heating time period after starting of heating of the article to be heated. This method relies on some kind of comparative temperature measurement, on that the heating process actually behaves as expected and that the products all have size and shape within relatively narrow intervals compared to the product used for the comparative temperature measurements. If something deviates from the expected ranges this method may indicate satisfactory surface temperature while the inside of the product may still be far from the desired temperature. It may especially be noticed that the operator may not even be aware of the fact that the inside has not reached the desired temperature.
GB2,145,245, on the other hand discloses an induction heating cooking apparatus adapted to be able to determine the temperature of the inside of the product. The apparatus is provided with a probe being inserted into the foodstuff to be cooked and detecting the temperature of the product. It is however often not acceptable to use a probe being inserted into the inside of the product. It is also difficult to use this kind of probing in an industrial process where a large number of products are treated simultaneously on a conveyor or the like.
EP0232802A1 discloses an apparatus for monitoring the cooking state of a substance, comprising an infrared light emitter and a sensor adapted to receive infrared light sent through product. The apparatus relies on that an alimentary substance being cooked varies its infrared light transparency, and thus the infrared light transmission and reflection coefficients, as the cooking process proceeds. As recognised in EP0232802A1 itself this method has its limitations and proposes that the light emitter and sensor are properly inserted into the substance to be monitored in order to give detailed results. As mentioned above is this kind of apparatus not satisfactory.
US2003024315, assigned to the present applicant, discloses a device for measuring the distribution of selected properties of materials. The device comprises an emitter of electromagnetic radiation and furthermore at least one sensor of a first type. The emitter emits electromagnetic radiation in a selected frequency range towards said materials and a sensor of the first type detects electromagnetic radiation in a selected frequency range coming from said materials. The detected electromagnetic radiation having been emitted by said emitter. The device also comprises means to generate a three-dimensional image contour information regarding the said material's position in space, and an analyser which (a) receives information from said sensors, (b) processes this information, and (c) generates signals containing information about the distribution of said properties as output.